The traditional way to make concrete in ready mix plants is to place sand, gravel and cement in separate hoppers feeding a drum or mixer which discharges into a concrete truck having a rotating drum to further mix en route to the job site where the cement is poured into the desired location, where it solidifies into concrete. Because the physical characteristics of each of the ingredients varies so much, traditional plants use powerful motors to mix, and the motors waste energy, and are inefficient. Typically, a batch will be twelve cubic yards of material premixed at the plant without water, and then placed in the concrete truck for further mixing by tumbling.
The prior art cement plants often do not have uniform mixing because of the large batch size and the differences in particle size among cement, fly ash, sand and aggregate. Water, if it is present, does not facilitate mixing, and sometimes hinders mixing when ingredients have hydrophobic surfaces. Also, the stiffness of the concrete affects the mixing characteristics. That is, high strength concrete has greater stiffness than low strength concrete, and the higher the strength, the more difficult it is to get complete mixing.
The strength of the concrete is measured by a variety of techniques, one of them is "slump," an analog of the strength of the concrete. Slump is determined by taking a cone-shaped receptacle filled with mixed, but not set, concrete 16 inches deep, turning the receptacle upside down on a horizontal surface, and releasing the cone. As the cone slumps when unsupported by the receptacle, it indicates the stiffness of the concrete. A slump of four inches, for example, indicates a relatively weak concrete, while a slump of one inch indicates a relatively strong concrete. The strongest concrete has zero slump. It will be apparent that a concrete that does not slump does not mix as readily as one that inherently oozes around the ingredients to achieve better mixing.
If there is inadequate mixing, there are pockets of unmixed materials that will be either voids or non-adhering particles. As a chain is only as strong as its weakest link, so too concrete is only as strong as the average strength of the set product. This has led designers of sky-scrapers, for example, to demand higher strength than would otherwise be called for in order to be sure the average strength meets the requirements. This overengineering leads to unnecessary costs that could be avoided by thorough mixing.
The desire for adequate mixing is also reflected in the mixing cycles. If a plant normally has a 60 second mixing cycle for a 12 yard batch, the operator will run the plant at 60 seconds even with a 8 yard batch, just to be sure the mixing is adequate. This means that the throughput of the plant is reduced with different sized batches. The result is a waste of energy and time.
Yet another mixing problem is that the size of aggregate varies widely, from about 3/16 inch to 1 and 1/2 inches. A great deal more energy is needed to mix large rock in making concrete, so plants are designed for the maximum energy requirements, even though those requirements are rarely needed.
In recent years, cement plants have used high intensity mixers, having two motors on the order of 100 horsepower each, to more thoroughly mix sand, cement, gravel and water before discharge into the concrete truck. Fly ash, a by-product of other industrial processes, is cheap and abundant, and may be substituted for up to 20% of the cement used in the typical ready mix plant. The motors are driven at peak power for a few seconds to charge the mixer with six cubic yards of ingredients, but the power is to some extent wasted, because the high energy consumption does not result in thorough mixing at the time of charging. After the first six yards are mixed and emptied from the mixer a second six yard batch is charged into the mixer. The motors are again driven at peak power to mix the second half of the 12 yard load on the truck. Again, the power is not optimally used.
Moreover, the high intensity mixers are practically limited to low strength concrete in transit mix operations, because the time needed to move stiff "low slump" concrete out of the premixer and into the truck is not compatible with the normal cycle times of the premixer. There is a bridging effect with low slump concrete in the truck, whereby the stiff concrete piles up and forms voids, with the result that the truck cannot hold a full load unless the concrete is added more slowly than usual. The capacity of the plant having high intensity mixers is reduced when high strength concrete is shipped.